A schematic stack structure of a general nitride semiconductor light emitting device and a fabrication method thereof will now be described.
A general nitride semiconductor light emitting device is provided in a stack structure including a substrate, a buffer layer, an n-GaN layer, an active layer and a pGaN layer sequentially stacked from a lower side.
In detail, in order to minimize the occurrence of crystal defects due to differences in the lattice constants and the thermal expansion coefficients of the substrate, for example, sapphire substrate, and the n-GaN layer, the buffer layer is formed of a GaN-based nitride or an AIN-based nitride having an amorphous phase at a low temperature. The n-GaN layer doped with silicon at a doping concentration of 1018/cm3 is formed at a high temperature as a first electrode contact layer. Thereafter, the growth temperature is lowered and the active layer is formed. Thereafter, the growth temperature is again elevated and the p-GaN layer doped with magnesium (Mg) is formed.
The nitride semiconductor light emitting device having the aforementioned stack structure is formed in a p-/n-junction structure which uses the n-GaN layer as the first electrode contact layer and uses the p-GaN layer as the second electrode contact layer.
To form the p-GaN layer used as the second electrode contact layer, the p-/n-junction light emitting device using the nitride semiconductor employs a doping source of Cp2Mg or DMZn. In the case of DMZn, since Zn is in ‘deep energy level’ within the p-GaN layer and has a very high activation energy, the hole carrier concentration serving as a carrier when a bias is applied is limited to about 1×1017/cm3. Accordingly, Cp2Mg MO (metal organic) having a low activation energy is used as the doping source.
In case the Mg-doped p-GaN layer is grown using a doping source of Cp2Mg, NH3 carrier gas and hydrogen (H) gas separated from the doping source are combined to form an Mg-H complex, which shows a high resistance insulation characteristic of more than ˜106Ω. Accordingly, in order to emit light during the recombination process of holes and electrons in the active layer, an activation process is essentially required to break the bond of Mg-H complex. Since the Mg-doped p-GaN layer has a high resistance, it can not be used without any change. The activation process is performed through an annealing process at an temperature range of 600˜800° C. in an ambient of N2, N2/O2. However, since Mg existing in the p-GaN layer has a low activation efficiency, it has a relatively high resistance value compared with the n-GaN layer used as the first electrode contact layer. In real circumstance, after the activation process, the atomic concentration of Mg in the p-GaN layer is in a range of 1019/cm3˜1020/cm3, and the hole carrier concentration contributing to a pure carrier conductivity is in a range of 1017/cm3˜1018/cm3, which correspond to a difference of maximum 103 times. It is also reported that the hole mobility is 10 cm3/vsec, which is a very low value. Due to the Mg atomic concentration remaining in the p-GaN layer without a complete activation, light emitting from the active layer toward the surface is trapped, or when a high current is applied, heat is generated due to a relatively high resistance value, so that the life time of the light emitting device is shortened to have a fatal influence on the reliability.
Especially, in the case of a large size/high power 1mm×1mm light emitting device using a flip chip technique, since a current of 350 mA which is very higher than a conventional current of 20 mA is applied, a junction temperature of more than 100° C. is generated at the p-/n-junction face, having a fatal influence on the device reliability and causing a limitation to product application in future. The generated much heat is caused by an increase of resistance component due to the Mg atomic concentration remaining in the p-GaN layer used as the second electrode contact layer without being activated as a carrier, and a rough surface property due to the increase of the resistance component.
Meanwhile, in the aforementioned p-/n-junction light emitting device, the n-GaN layer used as the first electrode contact layer can easily control the hole concentration within 5˜6×1018/cm3 within a critical thickness ensuring the crystallinity in proportional to the silicon doping concentration depending on an increase in the flow rate of SiH4or Si2H6, whilst in the p-GaN layer used as the second electrode contact layer, the hole concentration substantially serving as carriers is limited within a range of 1˜9×1017/cm3 although the flow rate of Cp2Mg is increased to dope Mg atoms of more than maximum ˜1020/cm3. To this end, the conventional light emitting device is made in a p-/n-junction structure having an asymmetric doping profile.
Thus, the low carrier concentration and high resistance component of the p-GaN layer used as the second electrode contact layer cause the light emitting efficiency to be decreased.
To solve the above problem, a conventional method of increasing the optical power by employing Ni/Au TM (transparent thin metal), having a good transmission and a good current spreading and a low contact resistance has been proposed, as a second electrode. However, the conventional method badly influences the device reliability when being applied to a large size/high power light emitting device. This problem still remains unsettled in the light emitting devices using the GaN semiconductor.